Investigating the relationship between corticosterone and glucose in a reptile

Abstract

The glucocorticoid hormone corticosterone (CORT) has classically been used in ecophysiological studies as a proxy for stress and energy mobilization, but rarely are CORT and the energy metabolites themselves concurrently measured. To examine CORT's role in mobilizing glucose in a wild reptile, we conducted two studies. The first study measured natural baseline and stress-induced blood-borne CORT and glucose levels in snakes during spring emergence and again when snakes return to the denning sites in fall. Our second study manipulated the hypothalamic pituitary adrenal axis in male snakes in the fall by taking a baseline blood sample, then subjecting individuals to one of five treatments (no injection, saline, CORT, adrenocorticotropin hormone, and metyrapone). Subsequent samples were taken at 30 minutes and 60 minutes. In both studies, we found that glucose levels do increase with the acute stress, but that the relationship was not directly related to CORT elevation. In the second study, we found that none of the HPA axis manipulations directly affected blood glucose levels, further indicating that CORT may play a complex, but not direct role in glucose mobilization in snakes. This study highlights the need for testing mechanisms in wild organisms by combining in situ observations with manipulative studies.

Glucocorticoids, energy metabolites, and immunity vary across allostatic states for plateau side‐blotched lizards (Uta stansburiana uniformis) residing in a heterogeneous thermal environment

“…Temperature differences in glucose mobilization may not occur for reptiles capable of thermoregulating within their environment considering glucose differences are most prominent during short-and long-term exposure to constant thermal stress (Callard et al, 1975;Gangloff et al, 2016;Ray & Maiti, 2001;Telemeco et al, 2017; but see . Despite glucocorticoids being suggested to mediate energetic balances in this context and those similar (e.g., Jacob & Oommen, 1992;Jessop et al, 2003;Neuman-Lee et al, 2020), we report no direct relationships between CORT and glucose. The role of CORT in regulating glucose levels may vary depending on the allostatic load experienced by reptiles and the remaining energetic stores they have available for use with facing additional challenges (Hudson et al, 2020;Neuman-Lee et al, 2020).…”

“…Despite glucocorticoids being suggested to mediate energetic balances in this context and those similar (e.g., Jacob & Oommen, 1992;Jessop et al, 2003;Neuman-Lee et al, 2020), we report no direct relationships between CORT and glucose. The role of CORT in regulating glucose levels may vary depending on the allostatic load experienced by reptiles and the remaining energetic stores they have available for use with facing additional challenges (Hudson et al, 2020;Neuman-Lee et al, 2020). The timing of release and action for CORT may also be key to if and how allostatic relationships with glucose are present.…”

“…As a primary mediator of allostasis, CORT in U. stansburiana uniformis seems to have shifted from modulating physiological processes relevant for daily activities to coping with an unpredictable acute stressor (Moore & Jessop, 2003). Similar patterns of allostatic mediation occur in several other reptiles (e.g., Jones & Bell, 2004;Moore et al, 1991;Neuman-Lee et al, 2020;Seddon & Klukowski, 2012;Tyrrell & Cree, 1998), with discrepancies assumedly arising from species-specific physiological and ecological factors.…”

“…Differences in energy balance between allostatic states for U. stansburiana uniformis were evident with shifts in circulating glucose, yet the stress‐induced glucose reductions that we found contrast trends typical of reptiles. Glucose mobilization often occurs during acute stressors of different types and durations (e.g., Franklin et al, 2003; Gangloff et al, 2017; Gregory & Schmid, 2001; Jessop et al, 2003; Neuman‐Lee et al, 2020; Telemeco et al, 2017), but sometimes no change seems to occur (Innis et al, 2014; Valverde et al, 1999). Granted reduced glucose has yet to be reported in the context of allostasis, we propose baseline concentrations are somehow sequestered for use during the acute stress response.…”

The relative speed of the glucorticoid stress response varies independently of scope and is predicted by environmental variability and longevity across birds

“…Moreover, the speed of GC regulation represents only a single component of speed in the more general stress response (Romero and Gormally, 2019). There has been increasing recognition in recent years that GC regulation alone is insufficient to understand variation in the stress response, because a greater GC response does not necessarily indicate a greater response in a variety of important downstream physiological or behavioral traits (Gormally et al, 2020; Neuman-Lee et al, 2020; Romero and Gormally, 2019). While these studies have generally focused on variation in scope, the same arguments apply to understanding variation in speed.…”